This proposal is concerned with the synthesis, structures, spectroscopic, and reactivity properties of specific Fe/M/S (M=Mo, V,W), Fe/S and Mn/O complexes. These compounds respectively are expected to serve as synthetic analogs for the Fe/M/S centers in the nitrogenases, the Fe2S2 centers in the Rieske proteins and the Mn4 cluster essential for water oxidation in photosynthesis. The great importance of metal centers in the function of metalloproteins and metalloenzymes is well recognized. The biochemical oxidation and reduction of substrates that occurs at metal containing active sites is intimately involved with crucial life sustaining processes. Synthetic analogs for the Fe/M/S center in nitrogenase (M=Mo, V) will be obtained with a Fe/M/S features, and spectroscopic characteristics similar to those in nitrogenase. A successful analog for the nitrogenase active site should allow for an understanding of the biological catalytic conversion (reduction) of dinitrogen to ammonia. This reaction also is potentially useful for our understanding of small molecule activation by metal complexes in general. The synthesis of specifically designed analog complexes for the Fe2S2, Rieske protein center will be important for the identification of the factors that affect the redox potentials of these centers. The synthesis of a decapeptide complex of the Fe2S2 center also will be carried out. The decapeptide ligand is designed to supply the amino acid residues thought to be present in the Rieske proteins. Comparative spectroscopic studies are expected to provide useful information regarding the Rieske protein centers. Very little is known about the structure of the Mn4 center involved in the oxidation of water during photosynthesis. Studies of the basic coordination chemistry of oligonuclear Mn clusters are needed prior to the design of specific analog complexes for the Mn center in photosystem II.